(a) Field of the Invention
The present invention relates to an inverter for a liquid crystal display.
(b) Description of the Related Art
Display devices used for monitors of computers and television sets include self-emitting displays such as light emitting diodes (LEDs), electroluminescences (ELs), vacuum fluorescent displays (VFDs), field emission displays (FEDs) and plasma panel displays (PDPs) and non-emitting displays such liquid crystal displays (LCDs) requiring light source.
An LCD includes two panels provided with field-generating electrodes and a liquid crystal (LC) layer with dielectric anisotropy interposed therebetween. The field-generating electrodes supplied with electric voltages generate electric field in the liquid crystal layer, and the transmittance of light passing through the panels varies depending on the strength of the applied field, which can be controlled by the applied voltages. Accordingly, desired images are obtained by adjusting the applied voltages.
The light may be emitted from a light source such as a lamp equipped in the LCD or may be natural light. When using the equipped light source, the total brightness of the LCD screen is usually adjusted using an inverter by regulating the ratio of on and off times of the light source or by regulating the current through the light source. The latter has a problem that the lighting for low brightness is unstable since the lamp current flowing in the lamp is very small. Since the former easily controls the amount of light, i.e., the luminance of the lamp without such a problem, the former is preferred.
However, the former has a problem called water fall that horizontal stripes slowly move upward and downward on the LCD screen unless the on/off frequency of the lamp is exactly equal to multiples of a frame frequency, i.e., a driving frequency of the LCD panel. For example, water fall moving with a frequency of 5 Hz is generated on the screen when the frame frequency and the on/off frequency are 60 Hz and 65 Hz, respectively. This phenomenon is a kind of beating and can be perceivable by human eyes even though the difference between the frequencies is as small as 0.1 Hz.